1. Field of the Invention
The present invention relates to a rotational or linear position detecting apparatus for mechanical products such as industrial machines and automobiles, jog dials of hand-held computing devices, and the like.
2. Description of the Related Art
Known rotary or linear pulse encoders each include a magnetic sensor and a ring or a linear scale facing the ring or linear scale and being magnetized so that the opposite magnetic poles can be alternately arranged at a regular pitch. The magnetic sensor translates the magnetic flux density in a direction passing through the magnetic sensor into Hall output voltage, the magnetic flux density changing along with relative rotation of the ring and the magnetic sensor or relative movement of the scale and the magnetic sensor. The translated analog signal is outputted as pulses through a Schmitt circuit or the like (for example, see ASAHI Hybrid Hall Effect IC EW-series catalogue by Asahi Kasei Electronics Co., Ltd. Jul. 1, 1999, 96303HE, P3, P6). Since the aforementioned linear scale is considered as a part of an alternately magnetized ring with infinite radius, the following description is given using a rotary pulse encoder as an example.
FIG. 3 is a view showing an example of a conventional rotary pulse encoder. Reference numeral 11 denotes a ring magnetized so that the opposite magnetic poles can be alternately arranged at a regular pitch. Reference numerals 12 and 13 denote Hall ICs as magnetic sensors placed near the ring. Herein, it is only necessary to count the number of pulses for detecting rotation angle or speed. In this case, only one sensor is used. FIG. 4 is a block diagram of each Hall IC used in such a case. The Hall IC includes a Hall element 21, am amplifier 22, a Schmitt circuit 23, and a driver, which are integrated. Such a Hall IC is widely used.
FIG. 3 shows the Hall elements 12 and 13 at a certain time when the Hall element 12 is located at the center of a south magnetic pole of and the Hall element 13 is located at the boundary between opposite magnetic poles. The shown arrangement of the Hall ICs relative to the magnetized ring illustrates a configuration to detect the rotation direction. This case is further described using FIG. 6. FIG. 6 is a view for clear explaining the relationship between the positions of the sensors relative to the magnetized ring, and the relationship between the positions and the magnetic flux density received by the sensors. In FIG. 6, the magnetized ring is drawn linearly. As shown in FIG. 6, at least two sensors (sensors A and B indicated by reference numerals 12 and 13, respectively) are placed at a distance equal to a half of the pole pitch of the magnetized ring. The rotation direction can be detected by observing an output of one of the sensors at a rising or falling edge of the output of the other sensor.
FIG. 7 is a view showing that the density flux densities detected by the Hall elements of the sensors A and B in FIG. 6 change with relative movement of the sensors A and B as the sensors A and B move or rotate relative to the magnetized ring at constant speed or constant angular speed. Herein, the point of t=0 indicates the positional relationship between the pole pitch of the magnetized ring and the sensors shown in FIG. 6. For example, as the sensor A is located at the center of a north magnetic pole when t=0, the magnetic flux density decreases in whichever direction the sensor A rotates. On the other hand, as the sensor B is located at the boundary between north and south magnetic poles when t=0, the sensor B moves toward the north or south magnetic pole depending on the moving direction. If the sensor B moves toward the north magnetic pole, the magnetic flux received by the sensor B changes to the same magnetic flux as the sensor A receives when t=0. If the sensor B moves toward the south magnetic pole, the magnetic flux received by the sensor B changes in an opposite way.
in FIG. 7, the output of the sensor B at each falling edge of the output of the sensor A is high during forward movement and is low during reverse movement.
The distance between the two sensors is preferably set so that the phase difference between the output signals of the two sensors is equal to an electrical angle of π/2. In this case, the positions of the sensors A and B depend on the pole pitch of the magnetized ring. When the phase difference is π/2, the rate of pulses is doubled by XORing the outputs of the two sensors, compared to the case of using one sensor. Accordingly, the detection resolution can be made twice as high as that obtained by only using the output of one sensor even with a same magnetized ring. In this case, it is ideally required that the sections with the opposite magnetic poles in the magnetized ring have same magnetic strength and a same dimension in the rotation direction and that the changes in magnetic flux received by the two sensors be equal to each other except for the phase. In such a case, the output of the aforementioned XOR operation has a duty ratio of 1:1, which has a constant period.
However, if the two sensors are separately arranged at a distance depending on the pole pitch of the magnetized ring so as to face the magnetized ring, the two sensors may be misaligned. Such mounting misalignment has a relatively great influence especially when the magnetized ring has a narrow pole pitch, in particular. In this case, it is difficult to enhance the detection accuracy. Specifically, when the pole pitch is narrow, the mechanical angle corresponding to the electrical angle of π/2 is small. Accordingly, the dispersions in mechanical positions of the two sensors more significantly affects the phase difference in electrical angle, or the optimal distance between the two sensors varies depending on the distances between the sensors and the magnetized ring.
To solve the aforementioned problems, a single package may be formed in such a way that the two sensors are formed on a same substrate at a distance and sealed in the single package, or that two sensors having same characteristics are formed on a same lead frame at a distance and sealed in the single package. However, magnetized rings to be sensed have a wide variety of sizes and pole pitches. For precise detection of magnetized rings having different pole pitches, it is necessary to change the distance between the two sensors depending on the magnetic rings to be sensed or according to the distances between the sensors and the magnetized ring.